This application relates to control and stabilization of lasers such as actively mode-locked lasers formed in fibers and other optical media.
Actively mode-locked lasers may include a laser cavity enclosing a laser gain medium to produce laser oscillation and an intra-cavity optical modulator to lock different laser modes supported by the laser cavity in phase to produce laser pulses. Either the amplitude or the phase of the intra-cavity optical field may be modulated at a frequency equal to a multiplicity of the mode spacing to achieve the mode locking. Such lasers may be designed to produce high-quality short optical pulses at a very high bit rate such tens of GHz or higher for various applications, including fiber communication systems.
The laser cavity in an actively mode-locked laser performs a number of functions. For example, the laser cavity provides an optical feedback mechanism to bounce light back and forth multiple times through the laser gain medium for optical amplification. The laser cavity also selects one or more proper lasing frequencies within the gain spectral profile of the laser gain medium by requiring each selected frequency of light to be in resonance with the laser cavity. As a further example, the laser cavity operates to confine the light rays of the laser approximately along the optic axis of the laser cavity by filtering out light rays in other directions. As a result, the geometry of the laser cavity affects various characteristics of the laser.
The geometry of the laser cavity, however, may change with respect to many factors, such as environmental conditions (e.g., temperature or vibrations) and its own aging process. One notorious factor is a change in the cavity length. Since a change in the laser cavity generally varies with time, it is therefore desirable to provide an active control mechanism to stabilize the laser cavity by dynamically adjusting the cavity.